Porous Metal Spine Implant Heals Bone

A highly porous metallic substance with characteristics similar to the spongy type of bone has been formed into a lumbar spine implant by Zimmer Holdings for use in the treatment of degenerative disc disease.

The company's new TM Ardis Interbody System is made of Zimmer's tantalum-based Trabecular Metal (TM) technology. The material has a structure, function, and physiology resembling those of cancellous, or trabecular, bone. This type of bone is softer than the other type, compact bone, and usually occurs inside vertebrae, near joints, and at the end of long bones.

The porosity and elasticity of Zimmer's TM material helps foster new bone growth between the implant and the patient's bone to speed healing. Zimmer already uses the material in its other products, made for use in the cervical spine in reconstructive orthopaedic applications, as well as in implants for the hips, knees, and extremities. The material has a history of several years of clinical success as an implant.

The TM Ardis Interbody System is a lumbar spine implant made with Zimmer's tantalum-based Trabecular Metal (TM) technology, a highly porous metallic substance with characteristics similar to the spongy type of bone. (Source: Zimmer)

TM has a porosity of up to 80 percent, with a consistent and open pore structure that is created by a nano-textured strut architecture. The elemental tantalum that TM is based on is biologically inert and chemically stable, with a strength higher than titanium. Tantalum has both a high fatigue strength and a compressive modulus that lets it bend before it breaks. The low modulus of elasticity of the TM material gives it a flexibility similar to that of cancellous bone.

Zimmer designed the TM Ardis implant with a large porous surface area. This surface area provides space for a maximum amount of bony in-growth, and also helps distribute load more evenly. The implant allows isoelastic load sharing, so it doesn't need as much stress shielding. It is shaped anatomically to make it easier to insert in a disc, and comes in 40 different sizes. According to a press release, this is "the first application of a porous metal implant with an interbody indication for the lumbar spine in the US."

The Trabecular Metal material starts as an engineered polyurethane foam, which is then transformed into pure carbon by being chemically treated and pyrolized. The carbon can be machined or crushed, and its pores remain open. Vitreous carbon components are then placed in a reactor where thermal deposition takes place. Pure tantalum is heated to very high temperatures and vaporized, then tantalum molecules in vapor attach to the carbon during chemical vapor deposition, building the TM into net shapes. This process creates a nanotextured surface of pure tantalum that bone and soft tissue can grow onto.

Perfect timing for the aging baby boomer generation which wants to stay active and fit, and is thus a regular fixture at the orthopedist. This type of implant seems like it could do wonders for the all hip replacements, knee replacements and other rites of passage this generation seems to be encountering given their commitment to staying youthful.

Wow. The fact that the application is for spine implants is impressive enough, but I'm impressed by the manufacturing process. Using a polymer substrate as a vapor deposition mold is awesome. --- Sort of like metal-infused ceramic, cermet, but with a polymer substrate - a "polymet" if you will. As we continue to see a miniaturization of electronics components, perhaps bio-compatible materials such as this will benefit from an increasing availability of tantalum...

I waited but it wasn't until the end they used the word "nano." I think this is key. To incorporate this material into the body permanently, it would seem that the merging of metal an tissue couldn't be on a surface only. It needs to be throughout the device just as the normal body parts are. Clever lads!

I hope this method also finds its way to other areas- broken bones, hips, etc. What a great idea!

warren, I had the same idea about "nano" regarding the material's ability to be incorporated in to the body. As the article states, the manufacturer has already used this material successfully for making implants and other products for hips, knees, and extremities, as well as in the cervical spine (in the neck). What's new here is the use for the lumbar spine (in the lower back), at least in the US.

Warren, I certainly agree. Being a "baby-boomer" myself, I can't tell you the number of family, friends, etc I know that could benefit from this technology RIGHT NOW. One very important sentence Ann put in her write-up--"THE MATERIAL HAS A HISTORY OF SEVERAL YEARS OF CLINICAL SUCCESS AS AN IMPLANT", indicates it could be ready for "prime time". I don't know the regulatory processes necessary for approval but if the technology is there, considerable suffering could be lessened and possibly eliminated. This is anoter great example of how engneering contributes to our society and how significant problems can be solved when proper engineering is applied. Great article Ann.

@williamlweaver: Actually, as Ann's article points out, the polyurethane is pyrolized before the CVD is carried out, so the tantalum is deposited on a carbon substrate. The resulting structure is said to be 99% tantalum and 1% carbon by weight.

Use polymer methods to create the substrate material. Use foaming methods to create the correct porosity and shape. Pyrolyze the polymer to create a carbon latice with the appropriate geometry and then use this latice as an engineered scaffold to assemble the tantalum via vapor deposition.

How freaking elegant is that!? =]

I agree my analogy to cermets is pretty loose. The resulting tantalum structure doesn't appear to get it's final mechanical properties as a polymer/metal composite.

I'm just wondering how soon this type of manufacturing process will be adopted for zeolite catalysts and other high-surface area materials like fuel cells. Maybe it already has... Great stuff!

I have to agree. Not only is the application itself very cool, but the development process itself is fascinating. I can almost see the future of active older adults careening down zip-lines and skydiving with impunity using their new tantalum-strengthed spines for support.

williamlweaver, I also found the manufacturing process fascinating, especially the use of vapor deposition processes, which I've encountered previously in electronics manufacturing contexts. I'm not sure if you intended this implication, but your comment sounds like you may be thinking of the possibility of extending the materials and/or processes described in the article to biocompatible materials in electronics. Is that an accurate guess?

I do wonder about the performance of this material (especially in terms of elasticity) over time. Human backs do a lot of bending, twisting and compressing. This is an application where you can't afford to have plastic failure. Any idea how long this would last, Ann?

Hi Chuck... I'm pretty sure the porous Tantalum substrate serves as a scaffold for new bone growth. Over time the metal implant becomes more natural bone than metal, providing the required mechanical and fatigue strength.

Chuck, good question, but I suspect we'd have to ask a surgeon that specializes in this surgery to find actual lifetime info for a given implant (or access the sugeons-only info on the manufacturer's website). Aside from replacing degenerated bone that has been removed, as williamlweaver points out, the main function of the device is apparently to support new bone growth, no easy task. As the manufacturer's website does say, the other implants made of this material have been used successfully for several years.

Focus on Fundamentals consists of 45-minute on-line classes that cover a host of technologies. You learn without leaving the comfort of your desk. All classes are taught by subject-matter experts and all are archived. So if you can't attend live, attend at your convenience.